<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .box{
      margin: 100px auto;
      width:80%;
      height:200px;
      display:flex;
    }
    .box div{
      height:100%;
    }
    .box div:nth-child(1){
      width:300px;
      background-color: #f00;
    }
    .box div:nth-child(2){
      flex: 1;
      background-color: #0f0;
    }
    .box div:nth-child(3){
      flex:2;
      background-color: #00f;
    }
  </style>
</head>
<body>
  <div class="box">
    <div>1</div>
    <div>2</div>
    <div>3</div>
  </div>
</body>
</html>